Austenitic steel alloy having excellent creep strength and resistance to oxidation and corrosion at elevated use temeratures

ABSTRACT

An austenitic steel alloy having excellent creep strength and resistance to oxidation and corrosion at elevated use temperatures up to approximately 750° C. has the following chemical composition (in weight %): 0.02≦C≦0.15%; 0.1≦Si≦2.0%; 25≦Cr≦33%; 22≦Ni≦38%; 1≦Mo≦6%; 0.4≦Nb≦1.5%; B≦0.0120%; 0.01≦N≦0.2%; Mn≦2%; Co≦5%; W≦2%; Al≦0.05%; Cu≦5%; Ti≦0.5%; Ta≦0.5%; V≦0.5%; P≦0.05%; S≦0.05%, Remainder iron with melting related impurities and optional addition of rare earths and reactive elements such as Ce, Hf, La, Re, Sc and/or Y of together 1%.

The invention relates to an austenitic steel alloy with excellent creep strength and oxidation and corrosion resistance at elevated operating temperatures according to patent claim 1 and work pieces made of the steel alloy according to claim 8.

In particular the invention relates to a heat resistant austenitic material for the production of pipes, steel sheets or as forging material for example for seamless super heater tubes in highly efficient power plants of the new generation which are suitable for steam temperatures up to about 750° C. The demands on the material under these conditions are a sufficient creep strength in combination with a good oxidation resistance in water vapor and corrosion resistance in the presence of flue gases and ashes.

For decreasing the CO₂ emission in power plants and for increasing the efficiency the steam boilers are impinged with ever higher steam temperatures and pressures. For improving the efficiency in the energy production in power plants there is therefore a demand to increase the steam temperature to up to 700° C. and above and also to increase the steam pressure in the boiler.

In particular attempts were undertaken in the past years to increase the steam pressure, which previously was about 600° C., to 650° C. or more and further to 700° C. or more.

The specific demands at the upper temperature levels on the heat exchange tubes at these high operating temperatures are a sufficient creep strength in particular in combination with a high oxidation resistance in water vapor and corrosion resistance in the presence of flue gas and ashes.

High temperature materials with high creep strength and corrosion resistance for use for example in power plants are generally based either on ferritic, ferritic/martensitic or austenitic iron based alloys or on nickel based alloys.

Chromium rich ferritic steel is compared to austenitic steel significantly more cost effective and has a higher thermal conductivity coefficient and a lower heat expansion coefficient. In addition chromium rich ferritic steels also posses a high oxidation resistance, which is advantageous for hot steam applications for example in heaters or in boilers.

However when a high temperature resistance, i.e., creep strength at high oxidation and corrosion resistance is required only austenitic steels for nickel based steel can be used.

Because nickel based alloys compared to austenitic steels are very expensive, there is a demand on the market for materials made of austenitic iron based alloys in particular pipes or pipelines which also offer the required creep strength and corrosion properties at high operating temperatures up to about 750° C. For example creep strengths of 10⁵ hours at 700° C. for a load of 100 MPa without fracture have to be achieved.

The known materials, which are available for operating temperatures up to about 620° C. or 650° C., are ferritic/martensitic steels with Cr-contents of for example 8 to 15%. These materials mostly have further expensive alloy additives or are not suited for the use in temperature ranges above 620° C.

Austenitic steels for use in steam boilers with steam temperatures up to 700° C. and above are for example known form DE 60 2004 002 492 T2. In this steel the creep strength is achieved in particular by adding titanium and oxygen in the stated limits. A disadvantage in this steel is however the insufficient oxidation resistance in water vapor and the insufficient resistance against flue gas corrosion at these high operating temperatures.

It is an object of the invention to set forth an alloy for an austenitic steel which reliably satisfies the state requirements regarding creep strength and oxidation and corrosion resistance also at operating temperatures up to about 750° C. and above.

A further object is to provide work pieces such as for example seamless or welded pipes, steel sheets and cast pieces or tool steels made of this steel alloy.

The first object is solved with the features of patent claim 1. Advantageous refinements are the subject matter of dependent claims. Work pieces according to the invention are set forth in claim 8.

According to the teaching of the invention a steel alloy with the following chemical composition (in weight %) is proposed:

0.02≦C≦0.15% 0.1≦Si≦2.0% 25≦Cr≦33% 22≦Ni≦38% 1≦Mo≦6% 0.4≦Nb≦1.5% B≦0.0120% 0.01≦N≦0.2% Mn≦2% Co≦5% W≦2% Al≦0.05% Cu≦5% Ti≦0.5% Ta≦0.5% V≦0.5% P≦0.05% S≦0.05%

Remainder iron with melting related impurities and optional addition of rare earths and reactive elements such as Ce, Hf, La, Re, Sc and/or Y of together 1%.

The austenitic ultra high temperature resistant alloy according to the invention has excellent creep strength properties and good oxidation resistance in water vapor and corrosion resistance in flue gas.

The alloy concept fundamentally differs from the known alloy concepts.

The strengthening of the austenitic matrix compared to dislocation creep occurs in known austenitic materials up to temperatures of 650° C. sufficiently by M₂₃C₆ on the grain boundaries and fine carbides and nitride particles in grain boundaries and in the grain interior. At higher temperatures no sufficient creep properties are ensured.

Tests have shown that for the strength failure under creep strength conditions of known austenitic materials at increased temperatures up to about 750° C. a weakening of the grain boundaries by the there precipitated sigma phase and associated with this a dissolution of the stabilizing carbides is essential. In addition the coarsening of the sigma phase precipitated in the grain after initial good creep strength behavior causes a fast strength decrease.

It was recognized as important for the invention that an improvement of the creep strength and the oxidation and corrosion resistance at increased temperatures can be only achieved by avoiding the above described effects of the grain boundary weakening and coarsening of the sigma phase precipitated in the grain.

An important aspect of the invention is therefore that the present alloy utilizes the fine particle sigma phase precipitated in the grain as strengthening component in combination with components M₂₃C₆ and further fine-particle carbides, carbonitrides and nitrides in particular niobium precipitated on the grain boundaries for increasing the creep strength.

The additional precipitation of the sigma phase in the interior of the grains while simultaneously suppressing these precipitations in the grain boundaries and a stabilization against coarsening of all precipitation types leads to an excellent creep strength up to 750° C.

Experiments have shown that corresponding to the above mentioned analysis a microstructure made of austenitic matrix with primary niobium carbides (Nb(C,N)) is generated after solution annealing for example at 1200° C./min quenching. After heat treatment at 700° C. or 740° C. for 4,000 h or in the creep strength test finely distributed sigma phase precipitations formed in the grain, and also small carbonitride precipitations of the type MX, wherein M is essentially niobium. A coarsening of the sigma phase was not observed up to testing times of almost 20,000 h. FIG. 1 shows the microstructure of the alloy according to the invention after annealing or creep strength test schematically.

This combination of properties is achieved according to the invention by a targeted adjusted to each other addition of chromium, molybdenum and silicone and carbon, niobium and nitrogen in the described ranges. Table 1 shows the tested materials. The steels not according to the invention serving as reference in the tests are marked with “X”.

TABLE 1 composition of the tested alloys (weight %) Not according to the Short name C Si Mn Co Cr Ni Mo W Nb B N invention AC66B 0.06 0.15 0.5 27 32 0.8 75 ppm 0.02 X AC66WB 0.08 0.15 0.5 27 32 4.5 1.5 0.8 75 ppm 0.02 MoW-I 0.08 0.15 0.5 27 32 3 1 0.8 50 ppm 0.02 MoW-II 0.06 0.15 0.5 27 32 3 1 0.6 50 ppm 0.02 MoW-III 0.08 0.15 0.5 27 32 2 1 0.8 50 ppm 0.02 MoW-I N 0.08 0.15 0.5 27 32 3 1 0.8 50 ppm 0.15 MoW-I Co 0.08 0.15 0.5 1.5 27 32 3 1 0.8 50 ppm 0.02 MoW-I W 0.08 0.15 0.5 27 32 3 0.8 50 ppm 0.02 X MoW-I NCo 0.08 0.15 0.5 1.5 27 32 3 1 0.8 50 ppm 0.15 MoW-I N oB 0.08 0.15 0.5 27 32 3 1 0.8 0.15 MoW-I Si 0.08 0.8 0.5 27 32 3 0 0.8 50 ppm 0.15 Sanicro 25 0.08 0.2 0.5 1.5 22.5 25 Cu: 3 3.6 0.5 50 ppm 0.23 X Alloy 617 0.06 0.08 0.06 12 22 rest 8.5 Fe: 1.5 Ti: 0.4 Al: 1 X

In order to ensure the strength increasing effect and the stability of the sigma-phase finely precipitated fine particles, a sufficient amount of fine particle sigma phase has to precipitate sufficiently fast at operating temperatures. According to the invention it is therefore provided that the sum content of molybdenum, chromium and silicone is at least 29 weight %.

For ensuring a sufficient stability and effectiveness of the other precipitated phases (Nb(C,N) and M23C6 it was discovered that the ratio in weight % (Nb(N+C) plays an important role. Tests have shown that a sufficient stability at increased operating temperatures is given when the ratio is in the range between 1.5 and 10.

An advantageous refinement of the invention provides that for further improving the corrosion resistance at high temperatures the minimal chromium content is set to 26% and the minimal nickel content to 25%. The increase of the lower limit of the chromium content achieves a higher chromium content in the austenitic matrix which significantly influences the oxidation and corrosion properties. The upper limit of the chromium content is lowered to 30% for limiting the content of sigma-phase. The nickel content is adjusted for stabilizing the austenitic structure. The upper limit can here be lowered to 35%, which results in a further improvement of the corrosion properties in sulfur containing flue gases and under sulfate containing layers.

Optimal properties regarding creep strength and corrosion can be adjusted with further narrowed content ranges of the elements. In this regard the contents of molybdenum are limited to 2-5% and that of silicone to 0.1-1% with regard to an optimal amount and distribution of the sigma phase. In addition the limitation of Nb(0.4-1%), N(0.05-1.12%) and C (0.05-1.12%) has a positive effect on the amount of niobium carbonitrides at high temperature (grain boundary pinning) on one hand and on the amount and distribution of M₂₃C₆ as well as other carbides, carbonitrides and nitrides at operating temperature on the other hand. The limitation of the upper limits also has a positive effect on a reduction of the tendency to segregations and on the processability of the steel.

DETAILED DESCRIPTION OF THE ALLOY CONCEPT

Carbon: the carbon content is a significant part of the alloy concept and serves for increasing the creep strength and yield strength by precipitation of carbides. A higher carbon content however decreases weldability. For this reason the upper limit is set to 1.15 weight % and the lower limit to 0.02 weight %.

Silicone: silicone is necessary in order to increase the corrosion resistance and to kinetically accelerate the precipitation of the sigma phase. A content of at least 0.1 weight % has proven advantageous. The weldability is negatively influenced by silicone, in addition silicone stabilizes the Laves-phase which is bound as a result of chromium precipitation so that an upper limit of 2 weight % should not be exceeded.

Manganese: Manganese is a cost effective element, which stabilizes the austenitic matrix of the alloy. In addition manganese decelerates the chromium loss of the alloy during oxidation in water vapor by evaporation of volatile chromium oxides in water vapor due to the formation of ternary Mn—Cr-Oxides. The Manganese content should on the other hand be kept low for avoiding accelerated oxidation in water vapor and flue gas. In addition an increased manganese content also negatively influences the creep strength. A content of maximally 2.0 weight % is not regarded as deleterious.

Chromium: the oxidation resistance on water vapor but in particular the resistance against flue gas corrosion is achieved by a chromium content of greater than 25 weight %. Chromium is also necessary for forming carbides M₂₃C₆ and for precipitation of fine particle sigma phase. Because the precipitation results in binding of chromium a content of at least 25 weight % is required in order to maintain the matrix concentration required to the corrosion resistance. In cooperation with molybdenum in the stated limits the dissolution of strengthening M₂₃C₆ carbides on the grain boundary in favor of more brittle sigma-phase is also prevented. At high chromium contents however increasingly d-ferrite forms which leads to a more coarse grained sigma phase. The maximal chromium content is therefore limited to 33 weight %.

Nickel: Nickel is a required element for achieving the austenitic structure and the strength advantages associated therewith, such as creep strength. The durability in sulfur containing flue gases is rather adversely affected by high contents of nickel so that at most 38 weight % nickel should be added. The lower limit should not fall below 22 weight % because due to the high chromium and molybdenum content the austenitic matrix is sought to be stabilized relative to the δ-ferrite.

Molybdenum: the addition of molybdenum to the alloy occurs for increasing the creep strength as a result of solid solution hardening. In addition a not too high content of molybdenum promotes the resistance against chloride containing gases and ashes. Molybdenum stabilizes beside M₂₃C₆ also the sigma phase and should therefore not fall below a minimal content of 1 weight %. According to the invention a molybdenum content of up to 6 weight % impedes in combination with chromium and boron the dissolution of strengthening M₂₃C₆ on the grain boundary in favor of the more brittle sigma-phase. At the same time molybdenum promotes precipitation of finely distributed sigma-phase in the grain for increasing the creep strength. Molybdenum contents of higher than 6 weight % cause the formation of an excessive content of sigma-phase and should also be avoided due to the segregation tendency of molybdenum.

Wolfram: wolfram can be added to the alloy as optional element and causes an accelerated oxidation in water vapor and corrosion under ash layers. Therefore the content should not exceed 2 weight %. At the same time wolfram causes an increase of the creep strength by solid solution hardening and formation of precipitations so that depending on the requirements a corresponding addition of wolfram can occur.

Niobium: the precipitation of hardening niobium carbides, niobium carbonitrides and niobium nitrides in the grain leads to a significant increase of the creep strength at operating temperatures. In addition wolfram based on the grain boundary pinning due to the Nb(C,N) precipitated on the grain boundaries, has a positive effect on the formation of a homogenous microstructure under production conditions. Higher contents of niobium however lead to segregations and decreased hot formability and weldability. The upper limit of 1.5 weight % should therefore not be exceeded. For an effective precipitation of carbides and nitrides at least 0.4 weight % are required. For an effective size of the precipitations the Nb, N and C contents have to be exactly adjusted to each other as described above.

Titanium, tantalum vanadium: precipitations that involve titanium, tantalum and/or vanadium can also lead to a significant increase of the creep strength. For decreasing accelerated oxidation or sulfur corrosion the upper limit is set however to respectively 0.5 weight %.

Boron: the addition of boron increases the creep strength due to a reduction of the tendency of increased coarseness and additional chemical stabilizing of M₂₃C₆-particles. In addition it increases the stability of grain boundaries against creep damage and increases ductility. Boron prevents the coarsening of the sigma phase by interfacial segregation and their precipitations on the grain boundaries. The lower limit for the effectiveness of boron is therefore about 0.0010 weight %. High boron contents adversely affect welding, which is why an upper limit of 0.0120 weight % is set.

Nitrogen: Nitrogen increases the creep strength as a result of precipitation of nitrides and therefore has to be added to the alloy as described above in dependence on the carbon and niobium content; nitrogen also stabilizes the austenitic matrix. The lower limit for nitrogen is therefore set to 0.01 weight %. A high nitrogen content causes a decreased tenacity and ductility and reduces the warm formability, therefore an upper limit of 0.2 weight % is set.

Cobalt: the optional addition of cobalt causes an increase of the solid solution hardening and with this the creep strength. Replacing nickel for cobalt is also conceivable for a sufficient stabilization of the austenitic matrix. At the same time the desired microstructure has to be maintained which is why an upper limit of 5 weight % is set.

Copper: copper can be added to the alloy and can be used as further hardening mechanism for the creep strength (precipitation of a Cu-phase). Higher contents of copper reduce the processability so that an upper limit of 5 weight 5% is set.

Rare earths and reactive elements; rare earths and reactive elements such as Ce, Hf, La, Re, Sc and/or Y can be optionally added at contents of together up to 1 weight percent for adjusting specific properties such as increased resistance to temperature changes.

FIG. 2 shows the excellent creep behavior at different operating temperatures of steels according to the invention compared to known steels, while FIGS. 3 and 4 show the time expansion behavior by way of expansion rates at 740 and 700° C.

Even though the steel alloy according to the invention can be used in the power plant field, its use is not limited thereto. Beside the production of pipes, which can be seamlessly extruded, hot and cold rolled or welded, this steel alloy can also be used for producing steel sheets, cast, forged and centrifugal casting parts or for tools for mechanical processing (tool steels), wherein their field of application includes pressure containers, boilers, turbines, nuclear plants or the chemical apparatus construction, i.e., all fields with corresponding demands at increased temperature.

Event though due to the excellent creep strength, corrosion and oxidation properties the steel alloy according to the invention can be advantageously used particularly up to temperatures of 750° C. or above, the use of this steel is already advantageous at temperatures above 600° C. when the focus is more on the strength of the material. 

What is claimed is:
 1. An austenitic steel alloy with excellent creep strength and oxidation and corrosion resistance at increased operating temperature up to about 750° C. with the following chemical composition (in weight %): 0.02≦C≦0.15% 0.1≦Si≦2.0% 25≦Cr≦33% 22≦Ni≦38% 1≦Mo≦6% 0.4≦Nb≦1.5% B≦0.0120% 0.01≦N≦0.2% Mn≦2% Co≦5% W≦2% Al≦0.05% Cu≦5% Ti≦0.5% Ta≦0.5% V≦0.05% P≦0.05% S≦0.05% Remainder iron with melting related impurities and optional addition of rare earths and reactive elements such as Ce, Hf, La, Re, Sc and/or Y of together 1%.
 2. The steel alloy according to claim 1, wherein the steel has the following composition (in weight %): Cr 26-30% Ni 25-35% B≦0.010%
 3. The steel alloy according to claim 1, wherein the steel has the following composition (in weight %): C: 0.05-0.12% Si 0.1-1% Cr 27-30% Ni 25-35% Mo 2-5% Nb 0.4-1.0% B max. 0.0090% N 0.05-012%
 4. The steel alloy according to claim 1, wherein a minimal content of B is 0.0010 weight %.
 5. The steel alloy according to claim 1, wherein for stabilizing a sigma-phase of the steel the steel satisfies the following condition: weight % Mo+weight % Cr+weight % Si≧29.
 6. The steel alloy according to claim 1, wherein for forming a sufficient amount of Nb(C,N) at simultaneous stability of M₂₃C₆ the steel satisfies the following condition: 1.5≦weight % Nb/(weight % N+weight % C)≦10.
 7. The steel alloy according to claim 1, wherein a microstructure of the steel under operating conditions at operating temperature has precipitated phases of M₂₃C₆ and further carbides, carbonitrides and nitrides on grain boundaries, and precipitated sigma phase, carbides, carbonitrides and nitrides in a grain of the steel.
 8. A seamless or welded steel pipe, steel sheet or work piece or tool steel produced by forging or casting with excellent creep strength and oxidation and corrosion resistance in particular at operating temperatures above 620° C., produced from the steel alloy according to claim
 1. 